The back of the fog lamp assembly is held in place by a torsional snap fit. To release it you should press on the lower beam which rotates and opens access to the back of the bulb.

The door in the lower panel is made in a cheap plastic (polypropylene which sells for about $0.30 per pound). When you rotate the knobs to open and close the door use a quarter and not a screw driver which may damage the knob.